EMPIR project develops digital twin for assessing optical power measurements

Image showing a light bulb and a stylistic representation of its digital twin
Stylistic representation of a digital twin

Software to assess losses and performance of Predictable Quantum Efficient Detectors for accurate and traceable optical power measurements

Photodiodes are used to measure the optical power or output of a light source. These operate in a similar manner to silicon solar cells, by converting the photons entering the instrument into electricity that in turn provides a readable output.

In a perfect detector each photon entering would trigger the release of one electron – giving a ‘quantum yield’ of exactly 1.0. This is the potential of an ideal Predictable Quantum Efficient Detector (PQED). These instruments are silicon trap detectors with photodiodes positioned in a wedge-shaped structure. They operate on principles that are tied to the fundamental constants of the universe, which makes these a very attractive candidate as a radiant power measurement standard. For this reason the mis en pratique, or methodology, for the realisation of the unit of light in the International System of Units (the SI) – the candela – includes PQEDs as one possible primary standard. These types of detectors can also deliver easy, cost effective, and accurate traceability for optical power measurements.

However, decreases in efficiency, caused by such things as the loss of electrons in the PQED due to material absorption, have been difficult to assess, reducing the usefulness of PQEDs.

The new digital twin software

Building on the iMERA- Plus and EMRP projects quantum Candela and NEWSTAR the EMPIR project  Self-calibrating photodiodes for the radiometric linkage to fundamental constants (18SIB10, chipS·CALe) developed a new improved PQED instrument along with the first PQED Digital Twin to exploit the use of the instrument as a primary standard.

The new 3D software has demonstrated an excellent agreement with experimental data at power levels from 100 μW to 1000 μW. The model also accurately predicts the responsivity at different wavelengths, beam sizes, power levels, temperature and bias voltages with many orders of magnitude changes in the internal quantum deficiency value.

Extensive characterisation of the PQED in the project also revealed a record high responsivity as predicted from the model, with one quantum-Candela PQED tested showing a quantum efficiency of 0.999970 with a relative expanded uncertainty of 0.000027. This is one of the lowest values ever achieved in spectral responsivity measurements of optical detectors.

The work on the PQED digital twin has been described in a Metrologia publication and the functionality of the PQED can be found in a video produced by the project.

Jarle Gran, from JV,  who coordinated the project said about the new digital twin software:

“The digital twin software enabled us to develop improved PQED photodiodes with record low internal losses as compared to the previous PQEDs and will be used as a reference to establish a new service for owners of PQEDs. The new service will support laboratories to independent realisations based on PQEDs alone. Prediction of PQED response over a wide spectral range, enabled by its Digital Twin, can be made from characterisations done at one wavelength with an extended characterisation interval due to the excellent stability. ”

Early uptake of the PQED

The Swiss institute for high mountain climate and medicine (SFI) Davos operates the World Calibration Centre for solar ultraviolet radiation on behalf of the World Meteorological Organisation. The current traceability chain of spectral solar UV irradiance to the SI is obtained via 1 kW tungsten halogen transfer standard FEL lamps. The stability of these lamps is a limiting factor of the whole measurement process, with an estimated uncertainty of the order of 0.5 %.

During the project SFI Davos constructed a PQED at JV, the National Metrology Institute of Norway. The PQED has been characterised for effects of UV radiation and initial experiments were promising and showed an agreement between photocurrent mode and electrical substitution mode within 200 ppm.

The data from the work is now planned to be used in the EURAMET Metrology Partnership project Self-calibrating photodiodes for UV and exploitation of induced junction technology (22IEM06, S-CALe Up), where radiation hardness will be tested and improved PQEDs for the ultraviolet spectral range will be developed and manufactured.

Jarle Gran, further commented:

“We have been able to extract the best out of the smart specialisation by National Metrology Institutes and external partners within the project. With the exploitation of detector and Digital Twin technology we have a low-cost fully predictable primary standard ready to be implemented as an absolute reference in a wide range of applications supporting health, environmental, quantum, space and industrial applications.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

EMRP joint research projects were part of EURAMET’s European Metrology Research Programme. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.


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